U.S. patent number 6,466,382 [Application Number 09/751,132] was granted by the patent office on 2002-10-15 for optical arrangement.
This patent grant is currently assigned to Carl-Zeiss-Stiftung. Invention is credited to Jochen Becker, Hubert Holderer, Wolfgang Hummel, Werner Muller-Rissmann, Rudolf Von Bunau, Christian Wagner, Stefan Xalter.
United States Patent |
6,466,382 |
Muller-Rissmann , et
al. |
October 15, 2002 |
Optical arrangement
Abstract
An optical arrangement, in particular a projection exposure
system for microlithography, has, in particular, a slit-shaped
image field or a non-rotational-symmetric illumination. As a
result, an optical element (101) is exposed in a
non-rotational-symmetric manner to the radiation of the light
source (110, 111, 112). The optical element (101) has an absorbing
coating (104, 105). The absorption of the coating (104, 105) is
distributed in such a manner that it is non-rotation-symmetrical in
a manner that is at least approximately complementary to the
intensity distribution of the exposure to the radiation (107, 108,
109) of the light source (110, 111, 112). As a result of the energy
absorbed in the coating (104, 105), an additional heating of the
optical element (101) takes place that results in a better
non-rotational-symmetric temperature distribution and,
consequently, a compensation for light-induced imaging errors.
Inventors: |
Muller-Rissmann; Werner
(Oberkochen, DE), Holderer; Hubert (Konigsbronn,
DE), Von Bunau; Rudolf (Essingen, DE),
Wagner; Christian (Aalen, DE), Becker; Jochen
(Oberkochen, DE), Xalter; Stefan (Oberkochen,
DE), Hummel; Wolfgang (Schwabach Gmand,
DE) |
Assignee: |
Carl-Zeiss-Stiftung
(DE)
|
Family
ID: |
7934875 |
Appl.
No.: |
09/751,132 |
Filed: |
December 29, 2000 |
Foreign Application Priority Data
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Dec 29, 1999 [DE] |
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199 63 588 |
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Current U.S.
Class: |
359/820;
355/70 |
Current CPC
Class: |
G03F
7/70058 (20130101); G03F 7/70216 (20130101); G03F
7/70891 (20130101); G03F 7/70958 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G02B 007/02 (); G02B 027/54 () |
Field of
Search: |
;359/820,642
;351/163,165,166 ;355/67,70 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A 198 07 094 |
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Aug 1999 |
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DE |
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B-0 532 236 |
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Jul 1997 |
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EP |
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Primary Examiner: Mack; Ricky
Attorney, Agent or Firm: Factor & Partners
Claims
What is claimed is:
1. An optical arrangement, comprising a projection exposure system
for microlithography, in particular having slit-shaped
illumination, having at least one light source that emits radiation
and an optical element that is heated by exposure to the radiation,
wherein the optical element is exposed to the emitted radiation of
the light source with non-rotational-symmetric intensity
distribution; wherein a) the optical element (1; 101) has an
absorbing coating (4, 5; 104, 105) having a spatial distribution
such that b) the absorption of the coating (4, 5; 104, 105) is
non-rotational-symmetric in an approximately complementary manner
to the intensity distribution of the exposure to the radiation (7,
107, 108, 109) of the light source (10; 110, 111, 112).
2. The optical arrangement as claimed in claim 1, wherein the
absorbing coating comprises at least two portions (4, 5) between
which there remains a non-absorbingly coated region (3) whose
dimension transversely to the axis of the radiation (7) is smaller
in at least one direction than the cross section of the radiation
(7) measured in the same direction.
3. The optical arrangement as claimed in claim 1, wherein the
absorbing coating (4, 5; 104, 105) has an absorptive power, varying
over its surface, for wavelengths of the radiation (7; 107, 108,
109) of the light source (10; 111, 112).
4. The optical arrangement as claimed in claim 3, wherein the
absorbing coating (4, 5; 104, 105) varies in its layer
thickness.
5. The optical arrangement as claimed in claim 3, wherein the
absorbing coating varies spatially in its absorption
coefficient.
6. The optical arrangement according to claim 3, wherein the
absorbing coating (4, 5; 104, 105) has the highest absorptive power
in the region that is nearest the centre of the surface of the
optical element exposed to the radiation.
7. The optical arrangement according to claim 1, wherein the light
source has a projection light source (110) and a compensating light
source (111, 112), wherein the radiation (108, 109) of the
compensating light source (111, 112) is directed at the absorbing
coating (104, 105).
8. The optical arrangement according to claim 1, wherein a spatial
distribution of the absorbing coating (104, 105) is such that
optical image errors of at least one other optical element are
compensated for by the non-rotational-symmetric absorption of the
radiation (7; 107, 108, 109) of the light source (10; 110, 111,
112) in an optical element.
9. An optical arrangement, comprising: a projection exposure system
for microlithography, in particular having slit-shaped
illumination, having at least one light source that emits radiation
and an optical element that is heated by exposure to the radiation,
wherein the optical element is exposed to the emitted radiation of
the light source with non-rotational-symmetric intensity
distribution; wherein a) the optical element (1;101) has an
absorbing coating (4, 5; 104, 105) having a spatial distribution
such that b) the absorption of the coating (4, 5; 104, 105) is
non-rotational-symmetric in a complementary manner to the intensity
distribution of the exposure to the radiation (7, 107, 108, 109) of
the light source (10; 110, 111, 112).
10. An optical arrangement, comprising: means for providing a
slit-shaped illumination; and an absorbing coating having a spatial
distribution such that the absorption of the coating is
non-rotational-symmetric.
Description
BACKGROUND OF THE INVENTION
The invention relates to an optical arrangement, in particular a
projection exposure system for microlithography, in particular
having slit-shaped illumination, having at least one light source
that emits radiation and an optical element that is heated by
exposure to the radiation, wherein the optical element is exposed
to the emitted radiation of the light source with
non-rotational-symmetric intensity distribution.
The imaging quality of such an optical arrangement is often reduced
by non-rotational-symmetric imaging errors. Such imaging errors
arise, for example, not only as a result of
non-rotational-symmetric light-induced heating of the optical
element, but also as a result of other light-induced effects, such
as, for example, "compaction", that result in a corresponding
non-rotation-symmetrical expansion or refractive-index distribution
in the optical element.
Given high requirements imposed on the imaging quality, such as
those demanded, in particular, in projection exposure methods in
microlithography, the light-induced imaging errors described cannot
be tolerated.
From the generic EP 0 678 768 A2, it is known to strive for an
improvement in the imaging properties by symmetrizing or
homogenizing the temperature distribution by an additional heating.
In this case, the additional heating takes place actively by means
of a plurality of heating elements that are thermally coupled to
the circumferential area of a lens. Such heating of the lens has
the disadvantage that the circumferential area of the lens has to
be heated relatively strongly to achieve, despite the thermal
conduction properties of the lens material, which are as a rule
poor, the desired symmetrization or homogenization of the
temperature distribution in the central region of the lens, which
is most relevant for the imaging properties. A strong heating of
the circumferential region of the lens results, however, in a risk
of damage to the lens and/or the lens mounting as a result of
thermal stresses.
Because of the relatively large spacing between the circumferential
region and the central region, which is modified by the radiation
of a light source serving for projection, a controlled structured
modifying of the temperature distribution in the vicinity of the
central region is, in addition virtually scarcely possible through
heating the circumferential region.
In the likewise generic EP 0 823 662 A2, optical elements are
additionally heated in order to compensate for the projection
light-induced imaging errors by coupling in additional light
sources that heat the optical elements by absorption at those
points where they are not irradiated by the projection light. Since
the heating power due to the additional light sources have to be
comparable to those of the projection light source to symmetrize or
homogenize the temperature distribution, for the additional light
sources, either light powers of the order of magnitude of those of
the projection light source are necessary or the additional light
sources must operate in wavelength ranges that are more heavily
absorbed by the material of the optical elements. In both cases,
expensive additional light sources are necessary. In the first
case, light sources having high power are necessary, and in the
second case, light sources having a wavelength are necessary that
are not available at low cost.
BRIEF SUMMARY OF THE INVENTION
The object of a the present invention is therefore to develop an
optical arrangement of the type mentioned at the outset in such a
way that a better symmetrization or homogenization of the
temperature distribution in the optical element can be achieved by
simple means.
This object is achieved, according to the invention, in that a) the
optical element has an absorbing coating having a spatial
distribution such that b) the absorption of the coating is
non-rotational-symmetric in an at least approximately complementary
manner to the intensity distribution of the exposure to the
radiation of the light source.
Such an absorbing coating results in an increased flexibility if a
specified additional heating of the lens is to be established by
means of the light absorption. New degrees of freedom, such as the
absorption coefficient and the form of the absorbing coating, are
then available for modifying the distribution of the additional
heating.
Preferably, the absorbing coating comprises at least two portions
between which there remains a non-absorbingly coated region whose
dimension transversely to the axis of the radiation is smaller in
at least one direction than the cross section of the radiation
measured in the same direction. As a result, the additional heating
can be brought about by means of the absorption of the projection
light itself. For this purpose, a smaller proportion of the
projection light is absorbed in the coating and results in the
desired symmetrization or homogenization of the temperature
distribution in the optical element by means of thermal conduction
in the coating and thermal coupling to the optical element. The
proportion of projection light that is absorbed in the coating is
in this case so small that the projection quality is virtually
unaffected thereby.
The absorbing coating has an absorptive power, varying over its
surface, for wavelengths of the radiation of the light source. As a
result, a more substantial adjustment of the spatial distribution
of the additional heating is possible by means of the light
absorption in the coating in order to symmetrize or homogenize the
entire temperature distribution of the optical element.
Such an absorptive power varying over the surface can be achieved,
for example, as follows:
The absorbing coating may vary in its layer thickness. This makes
possible coating with uniform material whose absorptive power
nevertheless varies over the surface.
Alternatively or additionally, the absorbing coating varies
spatially in its absorption coefficient. As a result, either a
coating is possible that has an absorptive power varying over the
surface with constant layer thickness, which is advantageous for
the production of anti-reflection layers, or an additional degree
of freedom is provided for the production of coatings varying in
absorption over their surface. In the latter case, relatively
complex absorption structures can also be produced. Such a
variation in the absorption coefficient can be achieved, for
example, by controlled doping of the absorption coating.
Preferably, the absorbing coating has the highest absorptive power
in the region that is nearest the centre of the surface of the
optical element exposed to the radiation. Additional heating
necessary for the symmetrization or homogenization then takes place
in the vicinity of the central region of the optical element, which
is the most strongly heated by the projection light, so that at
that point a strong additional heating can take place in a
controlled manner for the purpose of symmetrization or
homogenization. An unnecessary heating of more remote regions in
order additionally to heat the region in the vicinity of the
central region heated by the projection light by thermal conduction
is superfluous.
In a refinement of the invention, the light source has a projection
light source and a compensating light source, wherein the radiation
of the compensating light source is directed at the absorbing
coating. The absorption coefficient of the coating and the
compensating light source can be matched to one another in such a
way that compensating light sources having very low light powers
and a standard emission wavelength can be used that are
correspondingly less expensive and less complicated than in the
subject matter of the abovementioned EP 0 823 662 A2. Thus, the
absorption coefficient of the coating can be adapted, for example,
to the emission wavelength of the known inexpensive light sources,
such as laser diodes. An additional control of the temperature
distribution is possible via the surfaces of the optical element
exposed to compensating light and via the distribution of the light
power over the exposed surfaces. Optionally, the additional heating
can also be adapted to the requirements by adjusting the size and
the position of the surface exposed to the compensating light. This
is done by suitable alignment of the compensating light
sources.
The spatial distribution of the absorbing coating may be such that
optical image errors of at least one other optical element are
compensated for by the non-rotational-symmetric absorption of the
radiation of the light source in an optical element. Such an
overcompensation makes possible compensation for imaging errors of
a system of optical elements. Under these circumstances, only one
of said optical elements has to have an absorbing coating.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention are explained in greater
detail below by reference to the drawing; in the drawing:
FIG. 1 shows a plan view of a lens coated according to the
invention;
FIG. 2 shows a section along the line II--II of FIG. 1, which
figure additionally shows peripheral rays of a light beam acting on
the lens;
FIG. 3 shows a section along the line III--III of FIG. 1;
FIG. 4 shows the lens of FIGS. 1 to 3 with an alternative light
exposure; and
FIG. 5 shows a section along the line V--V of FIG. 4.
DETAILED DESCRIPTION OF THE INVENTION
The lens 1 shown in FIGS. 1 to 3 is part of an optical arrangement
for a projection exposure system for microlithography.
A main body 2 of the lens 1 is composed of a material, for example
quartz glass or CaF.sub.2, that has good transparency for the
wavelength range of the radiation of a projection light source 10.
The main body 2 is coated in three parts on an optical surface.
Two absorption layer segments 4, 5 are adjacent to a central
anti-reflection layer strip 3 in such a way that they jointly cover
one of the optical surfaces of the main body 2 except for a free
circumferential edge region 6 that is occupied in the assembled
state of the lens 1 by a mounting (not shown). Neither the layer
thicknesses of the anti-reflection layer strip 3 and of the
absorption layer segments 4, 5 nor the ratio of these layer
thicknesses to one another are shown to true scale in the
drawing.
The reflection of the lens 1 is minimized in the region of the
wavelength of the radiation of the projection light source 10 by
the anti-reflection layer strip 3.
The layer thickness of the anti-reflection layer strip 3 is
constant over its entire surface. The absorption layer segments 4,
5, on the other hand, have a continuous variation in their layer
thickness. In the region of that meridianal of the lens 1
(sectional plane of FIG. 2) that extends perpendicularly to the
anti-reflection layer strip 3, the layer thickness of the
absorption layer segments 4, 5 is greatest, whereas it continuously
decreases towards the edge of the main body 2 in the direction
perpendicular thereto (parallel to the sectional plane of FIG.
3).
Within the framework of the microlithography process, the lens 1 is
irradiated with a rectangular projection light beam 7 of the
projection light source 10. The cross-sectional area of the
projection light beam 7 is shown by broken lines in the region
where it passes through the lens 1 in FIG. 1 and has an aspect
ratio of the long side to the narrow side of approximately 2:1.
In the light exposure shown in FIG. 1, the narrow side of the
projection light beam 7 is wider than the width of the
anti-reflection layer strip 3, with the result that the edge
regions of the projection light beam 7 pass through the absorption
layer segments 4, 5.
The coated lens 1 functions as follows:
Because of the residual absorption of the material of the main body
2 in the region of the wavelength of the projection light beam 7,
the lens 1 is heated by the projection light beam 7, which has a
rectangular cross-sectional area in the region of the lens 1. This
heating initially has the symmetry of the light channel of the
projection light beam 7 in the main body 2. It results both in a
thermal expansion of the material and in a refractive-index change
and therefore, due to the changed refraction properties, in a
change in the imaging properties of the lens 1.
The object of the absorption of the radiation of the projection
light bundle 7 in the absorption layer segments 4, 5 is to achieve,
through a resultant additional heating in a manner complementary to
the heating due to the projection light beam 7, a symmetrization or
a specified form of the temperature distribution in the lens 1 that
results in well controllable imaging properties of the projection
exposure system.
With the light exposure, shown in FIG. 1, of the lens 1 to the
projection light beam 7, the additional heating takes place in
those regions in which the projection light beam 7 passes through
the absorption layer segments 4, 5. Since the material of the
absorption layer segments 4, 5 has a spatially constant absorption
coefficient, their absolute absorption is highest in the region of
the sectional plane of FIG. 2 in which the layer thickness of the
absorption layer segments 4, 5 is greatest. The strongest
additional heating of the main body 2 therefore takes place there
as a result of the absorbed energy in the absorption layer segments
4, 5.
This additionally absorbed energy distributes itself in the
absorption layer segments 4, 5 as a result of thermal conduction.
Heating of those regions of the main body 2 that are not exposed to
the projection light beam 7 is produced in the vicinity of the long
sides of the cross-sectional surface of the projection light beam 7
as a result of the thermal coupling of the absorption layer
segments 4, 5 to the main body 2.
In this connection, the layer thickness distribution of the
absorption layer segments 4, 5 is chosen in such a way that, as a
result of the two components of the heating, that is to say the
residual absorption in the main body 2, on the one hand, and the
absorption in the absorption layer segments 4, 5, on the other, a
temperature distribution in the main body 2 is produced which
approaches the desired rotational symmetry more closely than the
temperature distribution that would be established without
absorption layer segments 4, 5.
A variation in the absorption in the absorption layer segments 4, 5
can be achieved not only by a layer thickness variation, but also
by a spatial variation in the absorption coefficient of the
material of the absorption layer segments 4, 5. For this purpose,
for example, the absorption layer segments 4, 5 are appropriately
doped in those regions in which a higher absorption is desired. In
that case, the absorption layer segments 4, 5 may have a constant
layer thickness over their surface (not shown).
The optical surface of the lens 1 situated opposite the three-part
coating described has, as a rule, an anti-reflection coating (not
shown). Of course, both transmission surfaces may also have the
three-part coating.
A contribution to the symmetrization of the temperature
distribution in the main body 2 in the case of light exposure to a
projection light beam can also be achieved by an alternative
configuration of the surfaces of the absorption layer segments.
Instead of a symmetrization of the temperature distribution in the
main body 2, a controlled overcompensation of the imaging behaviour
of the lens 1 can also be achieved in that case with the aid of the
absorption layer segments 4, 5. If, for example, the lens 1 is part
of an optical arrangement comprising a plurality of adjacent
lenses, the imaging error produced by the asymmetrical heating in
the adjacent lenses can be compensated for by a controlled
overcompensation of the imaging properties of the one lens 1. For
this purpose, for example, an astigmatism in the main body 2 that
corresponds to that astigmatism, rotated approximately through
90.degree., that would arise solely as a result of the heating of
the other lenses in the optical arrangement by the residual
absorption in the main bodies 2 is produced by the additional
heating of the main body 2 of the one lens 1 by the absorption in
the absorption layer segments 4, 5.
The use of the absorption layer segments described is not
restricted to refractive optical elements, such as, for example,
the lens 1. Reflective optical components, such as, for example, a
mirror or a reflection grating, can also be provided with such
absorption layer segments in order to achieve in that case a
compensation for image errors through a controlled additional
heating as a result of the absorption of radiation of a projection
and/or a compensation light beam (cf. below).
An alternative light exposure of a lens 101 having absorption layer
segments 104, 105 is shown in FIGS. 4 and 5. Components that
correspond to those in FIGS. 1 to 3 bear reference symbols
increased by 100 and are not explained in detail yet again.
In FIG. 4, the entire cross-sectional area of the projection light
beam 107 passing through the lens 101 is situated within the area
of the anti-reflection light strip 103. The absorption layer
segments 104, 105 are exposed to compensating light beams 108, 109
from compensating light sources 111, 112 of, for example, laser
diodes.
In the case of the light exposure shown in FIG. 4, the additional
heating for the purpose of symmetrizing the temperature
distribution in the main body 102 takes place by the absorption of
the radiation of the two compensating light beams 108, 109. The
extent of the additional heating can in this case be adjusted, on
the one hand, by means of the absorption of the absorption layer
segments 104, 105 and, on the other hand, by means of the radiation
power of the compensating light beams 108, 109. This adjustment
takes place optionally in a controlled manner and as a function of
the signals of a sensor (not shown) that monitors the imaging
quality of the lens 1 or of the optical arrangement of the
projection exposure system. Such a sensor may be a two-dimensional
CCD array on which an image plane of the optical arrangement is
imaged.
* * * * *